This invention relates to the heating and cooling of room air spaces by thermal exchange with a heat store, and, more particularly, to such heating and cooling using a heat store emphasizing a high thermal mass of PCM material.
As used herein, the expression PCM is an abbreviation for phase change materials and is intended to denote a broad class of hydrated salts which undergo a fusion and crystalization phase transition at above comfortable room temperature, at about 80-90 degrees Fahrenheit. In the present application, PCM includes not only these salts but other materials which are useful for storing thermal energy. It is however more economical and more space conserving to utilize phase change materials than to use other thermal storage means. Preferred phase change materials include the known compounds as sodium sulfate decahydrate, which melts at 90.degree. F. and calcium chloride hexahydrate which melts at 80.degree. F. A heat store is a mass of PCM material supported and arranged in a convenient array for passing a heat exchange fluid therethrough.
The phenomena of overheating of buildings of relatively light mass by solar exposure or intermittent energy consuming processes carried out in such buildings is recognized and well documented. The solution of adding thermal mass to average out thermal excursions and to stabilize the temperature of such buildings is also known. The present invention is directed to incorporating a phase change material (PCM) as the thermal mass for this purpose in such a way as to render the PCM as highly effective and capable of both giving heat to building spaces within the building and of taking out heat from the room space.
Overheating of buildings is generally controlled by suitable ventilation, insulation, air conditioning and by proper placement, size, and shading of the glazing. Overheating can be caused by solar heat absorption in residential or office structures, and by lighting, occupancy, and process control in commercial manufacturing structures. However, during the heating season, excessive thermal energy gain can be advantageous if this energy is preserved for periods over which there is a heating demand. It is known that the addition of thermal mass to a structure can absorb and retain excess energy thereby stabilizing the temperature at a comfortable level. Various proposals for temporary storage of energy have been made. Reference is made to U.S. Pat. No. 4,037,650 wherein a thermal exchange between two thermal stores is achieved with a refrigeration heat transfer system room air is used in the heat exchange media in the external circuit. Heat is transferred between the stores in oen cycle of operation and is transferred to or from the room by operation of either selected one of the stores. This system is unduly complex, requiring separated stores of PCM, separate fans, and separate control circuits for heating and cooling. In its heat transfer cycle no set heating or cooling of room air takes place, while on entry heating and cooling only one-half of the thermal storage capacity is in use.
While it is also known to provide excess solar gain through southerly glazing, even in retrofit situations it is generally more complex and expensive to add a thermal mass, especially in the such retrofit situation. Consequently, overheating occurs in many structures. Or, if desired, it can be made to occur even in winter on cold sunny days. In general the overheated air in the building is circulated over the surface area of containers of stored thermal mass containing, for example, phase change materials (PCM) to absorb the excess energy for later use. Phase change materials are particularly suitable for this purpose; they have the ability to store very large amounts of energy at a given melting temperature by virtue of their high latent heat of fusion. However, such thermal mass cannot be effectively utilized with the generally small temperature differentials, less than about 20 degrees, which are usually to be encountered. In any event, the temperature variations which are sufficient to cause discomfort to persons are usually insufficient for the purpose of creating an adequate differential temperature for efficient heat transfer in systems. Thus, while a thermal storage unit for use in structures where overheating naturally occurs would be desirable, it is generally unavailable in any effective way. There is therefore a need for a new improved thermal storage system.